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Federer: I felt bad beating Andy Murray

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Sharapova, who won the French Open to highlight her 2012, bravely stayed in the contest by clinging on to her service, but at 3-5 down she cracked again.

Crunching ground strokes set up two championship points, finished with an explosive forehand as Williams claimed the WTA Championships for the third time.

Azarenka clinched the season-ending ranking by reaching the semifinals, but on current form Williams will return to the top of the rankings early in 2013.

"I made a little bit of a slow start to 2012," she told Eurosport after her Istanbul romp.

"There's always 2013 and I'm looking forward to it," she added ominously for the opponents at the elite end of the women's game.

Since mid-summer she has claimed the Wimbledon and U.S. Open titles before coming out on top in the WTA Tour's finale, despite having not played since her Flushing Meadows triumph.

With golds in the Olympic singles and doubles, with sister Venus, at the London Games, it has proved a glorious finish to the year.

Meanwhile, Juan Martin del Potro shocked World No.1 and home favorite Roger Federer in the final of the ATP tournament in Basel.

The Argentine did not drop his service throughout the three-set match to deny the Swiss maestro a sixth title at the hard court tournament.

He emerged a 6-4 6-7 7-6 winner after two hours and 44 minutes to claim back to back titles after winning in Vienna the previous week.

Federer, under pressure from Novak Djokovic for the top spot in the rankings, later withdrew from the Paris Masters this week where he is defending champion to rest up ahead of the ATP Tour Finals in London.

"I have some niggling stuff that I want to heal, I don't want to take a chance for London," he added.

In 2011 Federer won in Basel, Paris and London in a superb end to the season.

Top seed David Ferrer won the Valencia Open title with a 6-1 3-6 6-4 victory over Aleksandr Dolgopolov of the Ukraine.